Sounds like your close. From what your saying I think the G1100 IP which is also 192.168.1.1 is why it's not working. Take your computer and plug into LAN port of G1100 and log in.
Go to My Network/Network Connections/Network (Home/Office)/Settings
Go down to IP Address and change to 192.168.2.1 then Apply. From that point on you will have to open a new browser and the login for the G1100 will now be 192.168.2.1.
Disable then enable you network card to to obtain a new IP address from G1100 and make sure you can connect to internet. Your STBs will need a reboot so they can get an IP in the 192.168.2.1 range. When they come back up they should be good with IMG/VOD.